The present invention relates to structuring channels between a mobileservices switching center and base stations in mobile communications using ATM (Asynchronous Transfer Mode), and more particularly to a system which periodically transfers ATM cells generated for each of a plurality of call types by means of a frame consisting of multiple cell slots.
Establishing channels between a mobileservices switching center and base stations using ATM (Asynchronous Transfer Mode) has been conventionally implemented. In this case, it is necessary to send multiple connections of cells from a base station to the mobileservices switching center through the same channel. Sending ATM cells over the same channel presents various problems because the base station handles speech information from mobile stations with severe delay condition at low CBR (constant bit rate).
One of the conventional techniques sequentially transmits, under competitive control, a plurality of connections of ATM cells carrying the same call type over the same channel. This is shown in FIG. 1.
The transmission system under the competitive control as shown in FIG. 1 inevitably results in waiting delay for transmission regardless of the call types when the competitive control is carried out.
The object of the present invention is to reduce mismatch between the generation timing and transmission timing of a cell or waiting time for transmission as much as possible by periodically transmitting a frame consisting of cell slots prepared to send each of fixed length cells (ATM cells) bearing information about a call type of the same kind or call types different in information transfer rates and so on.
This makes it possible to minimize the delay from the generation to the transmission of the cells, and to ensure the quality in voice or real time information transmission.
According to a first aspect of the present invention, the cells of the same and different call types are transmitted periodically using a frame consisting of cell slots matching the cell generation timings which differ among the same and different call types. Using this frame can reduce the delay involved in sending out cells.
In the transmission above, if the number of calls exceeds the number of cell slots prepared for respective call types, an incomplete call will result because of the lack of transmission means.
According to a second aspect of the present invention, a cell slot which is conventionally a vacant slot and not used for information transfer is utilized as a shared cell slot available among the plurality of call types so that a fixed length cell (ATM cell) of an overflowed call is transmitted by the shared cell slot. Thus using the shared cell slot makes it possible to increase the information transfer efficiency of a channel.
The transmission of the overflowed call using the shared cell slot will sometimes result in an incomplete call regardless of a priority call or a non-priority call when the number of calls exceeds the number of available shared cell slots in the shared slots.
According to a third aspect of the present invention, the probability that the priority call ends with the incomplete call can be reduced by assigning the shared slot to the call in accordance with its priority in the overflowed calls, and by transmitting it.